Thermosiphon for electronics cooling with nonuniform airflow

ABSTRACT

A heat sink assembly for cooling an electronic device comprises a fan housed in a shroud, the fan including a hub and fan blades extending therefrom for causing an axially directed airflow through the shroud upon rotation of the fan blades. A thermosiphon comprises an evaporator defining an evaporating chamber containing a working fluid therein and further including a condenser mounted thereabove. The thermosiphon is positioned at one end of the shroud such that the fan is aligned with the condenser for directing the axial airflow therethrough. The condenser includes a plurality of tubes forming a tube grouping. Each tube having an opening in fluid communication with the evaporator and for receiving and condensing vapor of the working fluid received from the evaporator. The tubes are axially aligned with the airflow and are laterally positioned such that a lateral width of the tube grouping is approximately equal to a width of the hub and substantially in lateral alignment therewith.

TECHNICAL FIELD

The present invention relates to heat sinks in general, and moreparticularly to heat sinks for use in dissipating waste heat generatedby electrical or electronic components and assemblies.

BACKGROUND OF THE INVENTION

Research activities have focused on developing heat sinks to efficientlydissipate heat from highly concentrated heat sources such asmicroprocessors and computer chips. These heat sources typically havepower densities in the range of about 5 to 35 W/cm² (4 to 31 Btu/ft²s)and relatively small available space for placement of fans, heatexchangers, heat sinks and the like.

At the component level, various types of heat exchangers and heat sinkshave been used that apply natural or forced convection or other coolingmethods. The most commonly existing heat sinks for microelectronicscooling have generally used air to directly remove heat from the heatsource. However, air has a relatively low heat capacity. Such heat sinksare suitable for removing heat from relatively low power heat sourceswith power density in the range of 5 to 15 W/cm² (4 to 13 Btu/ft²s).Increases in computing speed resulted in corresponding increases in thepower density of the heat sources in the order of 20 to 35 W/cm² (18 to31 Btu/ft²s) thus requiring more effective heat sinks. Liquid-cooledheat sinks employing high heat capacity fluids like water andwater-glycol solutions are more particularly suited to remove heat fromthese types of high power density heat sources. One type of liquidcooled heat sink circulates the cooling liquid so that the liquidremoves heat from the heat source and is then transferred to a remotelocation where the heat is easily dissipated into a flowing air streamwith the use of a liquid-to-air heat exchanger. These types of heatsinks are characterized as indirect heat sinks.

As computing speeds continue to increase even more dramatically, thecorresponding power densities of the devices rise up to 100 W/cm². Theconstraints of the necessary cooling system miniaturization coupled withhigh heat flux calls for extremely efficient, compact, simple andreliable heat sinks such as a thermosiphon. A typical thermosiphoncomprises an evaporating section and a condensing section. Theheat-generating device is mounted to the evaporating section. In somethermosiphons, the heat-generating device is affixed to the internalsurface of the evaporating section where it is submerged in the workingfluid. Alternatively, the heat-generating device can also be affixed tothe external surface of the evaporating section. The working fluid ofthe thermosiphon is generally a halocarbon fluid, which circulates in aclosed-loop fashion between the evaporating and condensing sections. Thecaptive working fluid changes its state from liquid-to-vapor in theevaporating section as it absorbs heat from the heat-generating device.Reverse transformation of the working fluid from vapor-to-liquid occursas it rejects heat to a cooling fluid like air flowing on an externalfinned surface of the condensing section. The thermosiphon reliesexclusively on gravity for the motion of the working fluid between theevaporating and condensing sections. As for the motion of the coolingfluid on the external surface of the condensing section, a fluid movingdevice like an axial fan is employed.

Most electronics devices have high degree of non-uniformity built intothem. Thermal management of these devices is subject to two constraintsthat the thermal engineer must address. First, the heat flux generatedby the electronics device is highly non-uniform. Second, the aircirculated by the air-moving device like an axial fan is verynon-uniformly distributed. Most computer chips have their heatgeneration concentrated in a very small region in the core of the chip.For example, a typical 40×40 mm² computer chip has almost 80% of itstotal heat flux concentrated in its central 10×10 mm² surface. The heatflux distribution in a typical electronics device is shown schematicallyin FIG. 4. The second non-uniformity is attributed to the attachment ofthe air-moving device like an axial fan attached to the exterior of thethermosiphon. Axial fan has a large hub which acts as blockage toairflow. The airflow exiting the axial fan is highly concentrated in theperipheral region of the fan blades as shown in FIG. 5. The maximum airvelocity is in the tip region of fan blades. The velocity falls offsharply to zero in the central hub region. Under certain flow conditionsand blade angle, the local velocity at the root of the fan blade mayeven become negative, i.e., opposite to the direction of the predominantairflow.

The non-uniformity of airflow is far more pronounced in push modewherein the fan blows relatively cooler ambient air into the heatexchanger. In pull mode, on the other hand, the fan sucks relativelyhotter air from the heat exchanger. For a high heat load push mode isadvantageous when airflow rate is low. In order to attain flatterairflow profile entering the heat exchanger face a standoff distance ofat least three times the hub diameter is preferable between the fan andthe heat exchanger. However, because of packaging constraints only aboutone-fifth to one-quarter of the hub diameter standoff distance istypically available between the fan and heat exchanger. This is becausethe airflow at the heat exchanger face is non-uniform.

A limitation of the axial fan relating to smallness of the pressure riseacross the fan needs to be borne in mind. The curve of the pressure headdeveloped by the fan falls off very rapidly as the volumetric flow rateof air increases. In other words, the air exiting an axial fan cannotsustain a high-pressure drop through the fins. Therefore, managing theairflow through the heat sink at a low-pressure drop is a very importantconsideration in the design of a thermosiphon.

It is apparent from the foregoing considerations that from a system'spoint of view, the computer chip, heat sink and fan assembly areconstrained not only by very non-uniform heat flux but also bynon-uniform airflow capable of sustaining small pressure drop across theheat exchanger. Ideally, the airflow should be high in regions of highheat flux and low in regions of low heat flux. Overlaying FIGS. 4 and 5in push mode clearly reveals that the airflow distribution is oppositeto that ideally desired for better heat transfer. This is detrimental tothe functioning of a computer chip, as the chip junction temperaturebecomes high because of inadequate heat removal locally from the core ofthe chip. The thermal performance penalty attributed to thesenon-uniformities can be of the order of 25 to 50% compared to the casewith uniform heat flux and uniform airflow. Thus thermal solutionbecomes considerably more challenging when the heat flux as well as theairflow is non-uniform. The difficulty is compounded when the availableairflow rate is small. Therefore, careful attention must be paid to thefluid flow and heat transfer boundary conditions when developing thethermal solutions for the computer chips.

The compact thermosiphons intended to fit in a computer case requireboiling and condensing processes to occur in close proximity to eachother thereby imposing conflicting thermal conditions in a relativelysmall volume. This poses significant challenges to the process ofoptimizing the thermosiphon performance.

Thus, what is desired is a thermosiphon optimization process tointensify the processes of boiling, condensation and convective heattransfer at the external surface of the condenser while maintaining lowairside pressure drop.

SUMMARY OF THE INVENTION

One aspect of the present invention is a heat sink assembly for coolingan electronic device. The heat sink assembly comprises a fan housed in ashroud, the fan having a hub and fan blades extending therefrom forcausing an axially directed airflow through the shroud upon rotation ofthe fan blades. A thermosiphon is positioned at one end of the shroudsuch that the fan is aligned with the condenser for directing the axialairflow therethrough. The thermosiphon comprises an evaporator definingan evaporating chamber containing a working fluid therein and furtherincluding a condenser mounted thereabove. The condenser includes aplurality of tubes forming a tube grouping. Each tube having an openingin fluid communication with the evaporator and for receiving andcondensing vapor of the working fluid received from the evaporator. Thetubes are axially aligned with the airflow and are laterally positionedsuch that a lateral width of the tube grouping is approximately equal toa width of the hub and substantially in lateral alignment thereto.

Another aspect of the present invention is a condenser for a heat sinkassembly for cooling an electronic device. The heat sink assemblycomprises a base having an upper housing affixed thereto wherein theupper housing has open ends. A fan is mounted at one of the open ends,the fan having a hub and fan blades extending therefrom for causing anaxially directed airflow through the housing upon rotation of the fanblades. A plurality of tubes is positioned within the housing fortransmitting therethrough a vapor of a working fluid. The tubes define atube grouping such that the tubes are arranged in axial alignment withthe fan and laterally positioned such that a lateral width of the tubegrouping is approximately equal to a width of the hub and substantiallyin lateral alignment thereto.

These and other advantages of the invention will be further understoodand appreciated by those skilled in the art by reference to thefollowing written specification, claims and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heat sink assembly embodying thepresent invention, wherein an axial fan is arranged to draw cooling airthrough a thermosiphon.

FIG. 2 is an elevational cross-section view of the thermosiphon shown inFIG. 1 and taken along the line 2—2

FIG. 3 is an enlarged segment of the cross-sectional view of theboilerplate shown in FIG. 2.

FIG. 4 is a typical heat flux distribution of an electronic devicerequiring cooling.

FIG. 5A is a typical air velocity distribution just downstream of theaxial fan used in conjunction with a thermosiphon in push mode.

FIG. 5B is a typical air velocity distribution just upstream of theaxial fan used in conjunction with a thermosiphon in pull mode

FIG. 6 is an elevational cross-section view of a second embodimentthermosiphon.

FIG. 7 is an elevational cross-section view of a third embodimentthermosiphon.

FIG. 8 is an elevational cross-section view of a fourth embodimentthermosiphon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For purposes of description herein, the terms “upper”, “lower”, “left”,“rear”, “right”, “front”, “vertical”, “horizontal”, and derivativesthereof shall relate to the invention as oriented in FIG. 2. However, itis to be understood that the invention may assume various alternativeorientations and step sequences, except where expressly specified to thecontrary. It is also to be understood that the specific devices andprocesses illustrated in the attached drawings, and described in thefollowing specification, are simply exemplary embodiments of theinventive concepts defined in the appended claims. Hence, specificdimensions and other physical characteristics relating to theembodiments disclosed herein are not to be considered as limiting,unless the claims expressly state otherwise.

Turning to the drawings, FIG. 1 shows an air-cooled thermosiphon heatsink 10, which is one of the preferred embodiments of the presentinvention and illustrates its various components.

As illustrated in FIG. 1 a single axial fan 14 is housed in shroud 16and coupled to thermosiphon 12 through duct 18. The fan 14 could be apull or push type fan, however, a pull type of fan is preferred tominimize shadowing of the thermosiphon 12 by the fan hub 15. Theshadowing effect of hub 15 occurs over a lateral width 55 denoted bydimension “H” and substantially at a center of thermosiphon 12. Theshadowing effect of hub 15 is greater with a push type fan than a pulltype fan and reduces the airflow behind the hub and thereby interfereswith the heat transfer from thermosiphon 12 to the cooling air stream.

Although axial fan 14 is configured as a pull type fan, and therebyminimizes the shadowing effect, FIGS. 4, 5A and 5B illustrate thedifferences between the heat distribution of the device 8 to be cooledand the areas of maximum airflow of fan 14. As shown in FIG. 4, the heatdistribution of device 8 approximates a bell curve with the greatestheat at the area above the center of device 8. Conversely, the area ofmaximum airflow in push mode, as illustrated in FIG. 5A, appears as aninverse of the heat distribution, namely minimal airflow in the middleand maximum airflow at the outmost portion of the fan. In like manner,FIG. 5B illustrates the airflow in pull mode being similar to the pushmode airflow illustrated in FIG. 5A. Therefore, without anyenhancements, the fan generates maximum airflow over the minimum heatregions and low airflow over the regions of maximum heat.

FIG. 2 shows a sectional view of a preferred embodiment of thethermosiphon 12. Thermosiphon 12 comprises an evaporator 20 and acondenser 22 mounted thereabove.

The evaporator 20 comprises a baseplate 26 having a thickness 25 denotedby dimension “t” and sidewalls 24 about a periphery of baseplate 26. Thethickness 25 “t” of the evaporator base plate 26 is suitably chosenbased on an analysis of the particular boiling and heat transferconsiderations for a desired application. Electronic device 8 having amean width 9 denoted by dimension “z” is attached to a bottom surface 27of baseplate 26 using a heat conductive adhesive, also known as “thermalgrease”. Bottom surface 27 is preferably polished for attachment ofelectronic device 8 to enhance the thermal contact from device 8 tobaseplate 26.

An upper surface of baseplate 26 defines boiling surface 31 and can havea plurality of stud fins 28 formed thereon. Stud fins 28 are preferablymachined as an integral part of baseplate 26 for maximum heat transfer.The boiling surface 31 of baseplate 26 can also have a surface coating30 deposited thereon to enhance the boiling properties of boilingsurface 31. Surface coating 30 can comprise a sintered metal powder ofaluminum or copper.

Sidewalls 24 have a height 37 denoted by dimension “h” and have a bottomaffixed to baseplate 26. Sidewalls 24 can also be integrally formed withbase 26 as a single structure. An upper surface of sidewalls 24 definesan upper horizontal flange 29 about the periphery of evaporator 20 towhich the base 32 of condenser 22 is attached thereby definingevaporating chamber 36. The height of evaporating chamber 36 is alsorepresented by dimension “h”. Base 32 is preferably affixed to flange 29by one of brazing, welding or diffusion bonding to form a leakproofchamber 36. The flange joint between the sidewalls 24 and base 32 can beenhanced by means of a trunion groove type mating of the protruding andrecessed side of the flange prior to brazing or welding. A good jointcan also be enhanced by means of peripheral screws (not shown) fasteningbase 32 to sidewalls 24. The screws provide additional reinforcement andprevent leakage at high pressure. Evaporation chamber 36 is charged witha working fluid 38 through charging port 40 in base 32. Chamber 36 alsofunctions as a manifold to distribute saturated or super-heated vaporinto the hairpin condenser tubes 44.

Condenser 22 comprises base 32 and two hairpin condenser tubes 44.Hairpin tubes 44 are formed in an inverted “U” shape wherein each legthereof has a respective inlet end 43 extending through base 32 intoevaporation chamber 36. Inlet ends 43 are open and place an interior oftubes 44 in fluid communication with evaporation chamber 36. In thismanner, working fluid vapor formed as a result of boiling on the boilingsurface 31 can enter either end of hairpin tubes 44 and rise therein forthe ultimate dissipation of heat. Each of hairpin tubes 44 has a width45 denoted by the dimension “a”; a bend radius 48 at an upper endthereof denoted by the dimension “R”; and is positioned above the areaof high heat flux {dot over (q)}″ of device 8. Radius 48 (R) is selectedsuch that tubes 44 and their respective legs form a tube grouping behindfan hub 15 within hub 15 diameter 55 as denoted by dimension “H”. Thus,hairpin tubes 44 reside in the wake of hub 15 in the middle of thethermosiphon 12, and serve primarily as conduits for vapor flow betweenthe evaporator 20 and the condenser 22.

Tubes 44 have a minimal lateral tube spacing 46 denoted by dimension“e”. The properties of base 32, and the minimum distance permissible forforming slots to receive the tube ends therein govern tube spacing 46.The spacing 46 (e) between the tubes 44 serves as a high aspect ratiorectangular duct 60 of cross section e×D where D is the depth of tubesalong the direction of the axial airflow through thermosiphon 12.Central duct 60 has a low airside pressure drop compared to fins andhigh heat transfer coefficient approaching that of two infinite parallelplates. The airflow through the central duct 60 serves to condense someof the vapor on the bare side of tubes 44 though most of thecondensation is skewed on the finned side of tubes 44.

Two types of fins are used in condenser 22 of thermosiphon 12. Firstfins 50 having a height 51, denoted by dimension “q”, are intentionallyplaced outside of tubes 44 and are substantially inline with the fanblades of axial fan 14 where the airflow is high. Second fins 52 havinga height 53, denoted by dimension “p”, are placed in the low flow regiondirectly behind the hub 15 between the legs of each hairpin tube 44.Fins 50 and 52 are generally of a convoluted accordion configuration andhave their apexes bonded to the surface of tubes 44 or housing 34 theycontact. Housing 34 encases tubes 44, first fins 50 and second fins 52to direct and maintain the airflow from fan 14 thereover.

The preferred working fluid of thermosiphon 12 is a fluid such asdemineralized water, methanol or a halocarbon such as R134a (C₂H₂F₄).For a thermosiphon 12 utilizing R134a as working fluid 38, both theevaporator and condenser can be fabricated out of aluminum. However, analuminum evaporator or condenser cannot be used when water is theworking fluid in view of the corrosive effect of water on aluminum overtime. An all-aluminum construction has the benefit of reducedmanufacturing costs. Because of its low thermal conductivity, aluminumpresents a higher thermal resistance in comparison to copper. Therefore,an evaporator 20 constructed from aluminum is not suitable when the heatflux generated by the electronics device 8 is very high. Copper is thepreferred material of construction for evaporator 20 when the heat fluxgenerated by the electronics device 8 is very high. Copper also has thebenefit of usability for both R134a and water based working fluids 38,while aluminum is generally suitable only for an R-134a working fluid.

Based on theoretical and experimental study, the following dimensions ofthermosiphon 12 found to be optimal: the ratio of the width 45 of tubes44 to hub diameter 55 of fan 14 is expressed by the relationship0.08≦a/H≦0.25; the ratio of the height 53 of second fins 52 to hubdiameter 15 of fan 14 is expressed by the relationship 0.125≦p/H≦0.5;the ratio of the height 51 of first fins 50 to diameter 55 of hub 15 offan 14 is expressed by the relationship 0.15≦q/H≦0.5; and the ratio ofthe height 37 of evaporating chamber 36 to the height 57 of tubes 44 isexpressed by the relationship 0.075≦h/L≦0.25. The linear fin density ofeach fin strip ranges from 8 fins per inch to 20 fins per inch.

In use, as device 8 generates power and thus, heat, the heat sogenerated is transferred to baseplate 26. As baseplate and especiallyfins 28 increase in temperature, surface 30 becomes sufficiently hot tocause the working liquid covering the baseplate 26 to nucleate or boil.The working fluid vapor rises and enters hairpin condenser tubes 44. Theheated vapor contacts the sidewalls of tubes 44 and transfers thethermal energy in the vapor to the walls of tubes 44 and thereafter byconduction to convoluted fins 50 and 52. Axial fan 14 causes cooling airto flow primarily through convoluted first fins 50 and secondarilythrough second fins 52 and duct 60, convectively drawing heat therefrom.By removing thermal energy from the vapor, the vapor is cooled below itscondensation temperature and condenses on the interior walls of tubes44. The condensed liquid congregates and falls back through tubes 44 tothe pool of working fluid in vapor chamber 38 whereupon the process isrepeated.

Turning now to FIG. 6 another embodiment 112 of a thermosiphon isillustrated wherein like features according to the previous embodimentare identified with like numbers preceded by the numeral “1”. Indescribing thermosiphon 112 of FIG. 6, only the components that differfrom the components of thermosiphon 12 of FIG. 2 will be described belowsince the common components are already described with reference to FIG.2.

In the embodiment of FIG. 6, two different tube heights are used inorder to utilize the region shadowed by fan hub 15 for vapor flow. Ahairpin tube 144 has a width 145 denoted by dimension “a” and is bent toa small radius 148 denoted by dimension “R”. Hairpin tube 144 is placedsubstantially above the highest heat flux {dot over (q)}″ region (thecenter) of device 108. The intervening space between the innermost tubesegments of hairpin tube 144 is filled with third convoluted fins 172having a height 173 denoted by dimension “n”. Wide tube 170 has a heightslightly greater than hairpin tube 144 and is formed to envelop thehairpin tube 144 within its inverted U-shape. Ends 169 of tube 170extend through base 132 such that an interior of tube 170 is in fluidcommunication with vapor chamber 136 through either end 169. Wide tube170 has a width 171 denoted by dimension “b” which is generally larger,and thus less restrictive, than width 145 of tube 144. Second fins 152having a height 153 denoted by the dimension “p” extend between adjacentlegs of tubes 144 and 170. Enveloping the tube 144 by tube 170 in thisfashion helps to maintain structural integrity at high internal pressureand also facilitates manufacturing.

By selecting third convoluted fin 172 having a height 173 and tube 144having a small bend radius 148, wide tube 170 can be kept relativelyclose to device 108. The top of wide tube 170 can also be angled fromthe horizontal to prevent condensate build up and thus, always ensurethe condensate return from the top of tube 170 to the chamber 136. Thesize of the hairpin tube 144 having bend radius 145 and the short height173 of fins 172 is selected specifically to utilize the low airflow inthe region of hub 115. Strategic placement of wide tube 170 on theoutside of tube 144, but within the width 155 of fan hub 115, enablesheat dissipation through first fins 150. The majority of the vaporgenerated in vapor chamber 138 flows through the less restrictive widetube 170 with larger cross-section and hence with lower flow resistance.First fins 150 are bonded to wide tube 170 and shroud 134 and arepositioned in the wake of the fan blades of fan 114, therefore ensuringgood airflow and lower overall airside pressure drop. In this fashion,fins 150 are placed in the periphery of thermosiphon 112 and areutilized to dissipate the majority of the latent heat from the vaporcarried by tube 170.

For the embodiment illustrated in FIG. 6 as thermosiphon 112, andthrough careful design and test iterations, it was established that thebenefits of the present embodiment are best realized within thefollowing ranges of the key dimensions: The ratio of the width 145 oftube 144 to hub diameter 155 of fan 114 is expressed by the relationship0.08≦a/H≦0.2. The width of wide tube 170 to hub diameter 155 isexpressed by the relationship 0.125≦b/H≦0.5. The ratio of the height 153of third fins 152 to hub diameter 155 of fan 114 is expressed by therelationship 0.08≦n/H≦0.4. The ratio of the height 151 of first fins 150to diameter 155 of hub 115 is expressed by the relationship 0.2≦q/H≦0.5.The ratio of the height 153 of second fins 152 to diameter 155 of hub115 is expressed by the relationship 0.08≦p/H≦0.375. The ratio of theheight 137 of evaporating chamber 136 to the height 157 of tubes 144 isexpressed by the relationship 0.075≦h/L≦0.25. The linear fin density ofeach fin strip ranges from 8 fins per inch to 20 fins per inch.

Turning now to FIG. 7 another embodiment 212 of a thermosiphon isillustrated wherein like features according to the previous embodimentare identified with like numbers preceded by the numeral “2”. Indescribing thermosiphon 212 of FIG. 7, only the components that differfrom the components of previous embodiments of FIGS. 2 and 6 will bedescribed below since the common components are already described withreference to previous embodiments.

The embodiment illustrated in FIG. 7 employs two hairpin tubes 244within a wide tube 270. Hairpin tubes 244 are positioned directly overdevice 208 where the maximum heat flux {dot over (q)}″ region isrealized. Wide tube 270 encompasses both hairpin tubes 244 and generallyextends the width of fan 214. Thermosiphon 212 fully addresses thenon-uniformity of the airflow.

Fin sizes as well as the linear fin densities are varied to conform tothe airflow induced by the fan. Third fins 272 having a height 273denoted by dimension “n” are placed between two closely spaced hairpintubes 244. Second fins 252 are medium sized having a height 253 denotedby dimension “p” and are positioned interiorly of the legs of eachhairpin tube 244. First fins 250 having a height 251 denoted bydimension “q” are positioned between hairpin tubes 244 and wide tube 270in the region corresponding to the maximum airflow from fan 214. Outerfins 280 extend between tube 270 and shroud 234 outside of the primaryairflow stream of fan 214. Fins 280 are of medium size and have a height281 denoted by dimension “r”.

This design is suitable for high heat load as well as for high heatflux. By employing non-uniform fins sizes, the pressure drop registeredby the flowing air from fan 214 is utilized profitably for carryingwaste heat. If the fins were of uniform size and density, the pressuredrop would have still occurred, however, the heat pick up would havebeen less due to a reduced availability of vapor flow rate at theperiphery. Selecting small bend radii and fins having a correspondinglyrelatively small height permits concentrating a maximum of tube spacedirectly above the core of the heat-generating device 208. In this way,the tube entrance losses are minimized for vapor flow and therebymaintaining an overall low vapor side pressure drop. As evident fromFIG. 7, tubes 244 are bundled behind the fan hub 215 and significantportion of the finned area is placed behind the blades of fan 214.Additional modulation of the airflow to qualitatively mimic the heatflux profile can be achieved by lowering the fin density in the middleand increasing the fin density at the periphery.

For the embodiment illustrated in FIG. 7 as thermosiphon 212, andthrough careful design and test iterations, it was established that thebenefits of the present embodiment are best realized within thefollowing ranges of the key dimensions. The ratio of the width 245 oftube 244 to hub diameter 255 of fan 214 is expressed by the relationship0.08≦a/H≦0.25. The width 271 of wide tube 270 to hub diameter 255 isexpressed by the relationship 0.08≦b/H≦0.2. The ratio of the height 253of second fins 252 to hub diameter 255 of fan 214 is expressed by therelationship 0.1≦p/H≦0.3. The ratio of the height 281 of outer fins 280to diameter 255 of hub 215 is expressed by the relationship 0.1≦r/H≦0.2.The ratio of the height 251 of first fins 250 to diameter 255 of hub 215is expressed by the relationship 0.2≦q/H≦0.4. The ratio of the height237 of evaporating chamber 236 to the height 257 of tubes 244 isexpressed by the relationship 0.075≦h/L≦0.25. The linear fin density ofeach fin strip ranges from 8 fins per inch to 20 fins per inch.

Turning now to FIG. 8 another embodiment 312 of a thermosiphon isillustrated wherein like features according to the previous embodimentare identified with like numbers preceded by the numeral “3”. Indescribing thermosiphon 312 of FIG. 8, only the components that differfrom the components of previous embodiments of FIGS. 2 and 6-7 will bedescribed below since the common components are already described withreference to previous embodiments.

As illustrated in FIG. 8, the hairpin tube 144 of thermosiphon 112 asillustrated in FIG. 6 has been combined into a single central stem tube396 in thermosiphon 312. The single tube 396 reduces the number ofbrazing joints and thereby further reduces the potential for leakage ofthe working fluid from the thermosiphon 312 since tube 396 has only oneinlet 395 extending through base 332 into evaporating chamber 336.Thermosiphon 312 utilizes different tube and different fin sizes. Thecentral stem tube 396 has a width 397 denoted by dimension “c” of widercross-section than previous tube 44. Central stem tube 396 is placedcentrally behind fan hub 315 and directly above the high heat fluxregion of device 308. Tube 396 is sealed at its top. Wide tube 370 has awidth 371 denoted by dimension “b” and is formed to have a substantiallyflat top over the top of central stem tube 396. First fins 350 at theperiphery have a height 351, denoted by dimension “q”, and aresubstantially in line with the airflow from fan 314. First fins 350 aregenerally of the same height or taller than second fins 352 having aheight 353 denoted by dimension “p”.

Thermosiphon 312 is particularly suited for high heat flux and veryconcentrated heat loads, and where spreading of heat is difficult andthe vapor side pressure drop requirement is low. Additionally, centralstem tube 396 significantly enhances heat transfer performance of theevaporator as a result of condensate dripping into the liquid pool 338directly over the center of device 308. This improves the performance ofthe boiling surface at very high heat flux.

For the embodiment illustrated in FIG. 8 as thermosiphon 312, andthrough careful design and test iterations, it was established that thebenefits of the present embodiment are best realized within thefollowing ranges of the key dimensions. The ratio of the width 397 oftube 396 to hub diameter 355 of fan 314 is expressed by the relationship0.125≦c/H≦0.3. The width 371 of wide tube 370 to hub diameter 355 isexpressed by the relationship 0.08≦b/H≦0.2. The ratio of the height 353of second fins 352 to hub diameter 355 is expressed by the relationship0.1≦p/H≦0.325. The ratio of the height 351 of first fins 350 to diameter355 of hub 315 is expressed by the relationship 0.2≦q/H≦0.5. The ratioof the height 337 of evaporating chamber 336 to the height 357 of widetube 370 is expressed by the relationship 0.1≦h/L≦0.375. The linear findensity of each fin strip ranges from 8 fins per inch to 20 fins perinch.

In the foregoing description those skilled in the art will readilyappreciate that modifications may be made to the invention withoutdeparting from the concepts disclosed herein. Such modifications are tobe considered as included in the following claims, unless these claimsexpressly state otherwise.

We claim:
 1. A heat sink assembly for cooling an electronic device, saidheat sink assembly comprising: a fan housed in a shroud, said fanincluding a hub having a diameter “H” and fan blades extending therefromfor causing an axially directed airflow through said shroud uponrotation of said fan blades; and a thermosiphon comprising an evaporatordefining an evaporating chamber containing a working fluid therein and acondenser mounted thereabove, said thermosiphon positioned at one end ofsaid shroud such that said fan is aligned with said condenser fordirecting said axial airflow therethrough; wherein: said condenserincludes a plurality of tubes forming a tube grouping, each said tubehaving an opening in fluid communication with said evaporator and forreceiving and condensing vapor of said working fluid from saidevaporator, said tubes being axially aligned with said airflow andlaterally positioned such that a lateral width of said tube grouping isapproximately equal to a width of said fan hub and substantially inlateral alignment therewith.
 2. The heat sink assembly according toclaim 1 wherein said evaporator includes a baseplate and has anelectronic device to be cooled mounted on a bottom surface thereof. 3.The heat sink assembly according to claim 2 wherein said baseplate hasan upper surface defining at least a portion of said evaporation chamberand includes a plurality of fins on said upper surface, said finsextending into said evaporating chamber.
 4. The heat sink assemblyaccording to claim 3 wherein said upper surface and said fins have arough surface coating thereon.
 5. The heat sink assembly according toclaim 4 wherein said rough surface coating comprises a sintered metallayer.
 6. The heat sink assembly according to claim 2 wherein saidbaseplate has an upper surface defining at least a portion of saidevaporation chamber and has a rough surface coating thereon.
 7. The heatsink assembly according to claim 6 wherein said rough surface coatingcomprises a sintered metal layer.
 8. The heat sink assembly according toclaim 2 wherein said tubes have a height dimension “L”, and saidevaporator has a height dimension “h” such that the ratio h/L fallswithin the expression 0.075≦h/L≦0.375.
 9. The heat sink assemblyaccording to claim 2 wherein said tube openings are substantiallyvertically aligned over said electronic device.
 10. The heat sinkassembly according to claim 1 further including a primary convoluted finaffixed to each of outermost tubes of said tube grouping and laterallyextending substantially to a tip diameter of said fan blades.
 11. Theheat sink assembly according to claim 1 wherein said tubes have a widthdimension “a”, such that the ratio a/H falls within the expression0.08≦a/H≦0.25.
 12. The heat sink assembly according to claim 1 furtherincluding a primary convoluted fin affixed to each of outermost tubes ofsaid tube grouping and laterally extending outwardly therefrom whereinsaid primary convoluted fin has a height dimension of “q” such that theratio q/H falls within the expression 0.15≦q/H≦0.5.
 13. The heat sinkassembly according to claim 12 wherein said primary convoluted fin has afin density falling within the range of 8-20 fins per inch.
 14. The heatsink assembly according to claim 12 wherein at least a first of saidtubes has an inverted U-shape having two downwardly extending legs andfurther wherein each leg has an opening in fluid communication with saidevaporating chamber.
 15. The heat sink assembly according to claim 14wherein said first of said tubes has said diameter dimension “b”, suchthat the ratio b/H falls within the expression 0.08≦b/H≦0.5.
 16. Theheat sink assembly according to claim 14 wherein at least a second ofsaid tubes is positioned within said U-shape of said first tube.
 17. Theheat sink assembly according to claim 16 wherein said at least secondtube has an inverted U-shape having two downwardly extending legs andfurther wherein each leg has an opening in fluid communication with saidevaporating chamber.
 18. The heat sink assembly according to claim 17wherein adjacent ones of said legs of said first and second tubes have asecondary convoluted fin extending therebetween.
 19. The heat sinkassembly according to claim 18 wherein said secondary convoluted fin hasa height dimension of “p” such that the ratio p/H falls within theexpression 0.08≦p/H≦0.5.
 20. The heat sink assembly according to claim19 wherein said primary convoluted fin and said secondary convoluted finhave a fin density falling within the range of 8-20 fins per inch. 21.The heat sink assembly according to claim 19 wherein said primaryconvoluted fin has a fin density different than said secondaryconvoluted fin.
 22. The heat sink assembly according to claim 17 whereintwo innermost ones of said legs of said first and second tubes define anunobstructed duct therebetween.
 23. The heat sink assembly according toclaim 17 further including a third convoluted fin extending between twoinnermost ones of said legs of said first and second tubes.
 24. The heatsink assembly according to claim 16 wherein said second tube comprises acentral stem tube having a single inlet in fluid communication with saidevaporator.
 25. The heat sink assembly according to claim 24 whereinsaid central stem tube has a width dimension “c”, such that the ratioc/H falls within the expression 0.125≦c/H≦0.3.
 26. The heat sinkassembly according to claim 11 further including an outer tubesurrounding said tube grouping and having an inverted U-shape with twodownwardly extending outer legs, an end of each leg having an opening influid communication with said evaporating chamber, each said outer legaffixed to an outer end of an adjacent one of said primary convolutedfins.
 27. The heat sink assembly according to claim 26 further includingan outer convoluted fin extending outwardly from each of said legs ofsaid outer tube.
 28. The heat sink assembly according to claim 27wherein said outer convoluted fin has a height dimension of “r” suchthat the ratio r/H falls within the expression 0.1≦r/H≦0.2.
 29. Acondenser for a heat sink assembly for cooling an electronic device,said condenser comprising: a base having an upper housing affixedthereto, said upper housing having open ends; a fan mounted at one ofsaid open ends, said fan including a hub having a diameter “H” and fanblades extending therefrom for causing an axially directed airflowthrough said housing upon rotation of said fan blades; a plurality oftubes within said housing for transmitting therethrough a vapor of aworking fluid, said tubes defining a tube grouping, said tubes beingarranged in axial alignment with said fan hub and laterally positionedsuch that a lateral width of said tube grouping is approximately equalto said diameter of said hub and substantially in lateral alignmenttherewith.
 30. A condenser according to claim 29 wherein each said tubedefines at least one opening for receiving into said tube a vapor of aworking fluid and for discharge of condensed working fluid.
 31. Acondenser according to claim 30 further including a primary convolutedfin affixed to each of outermost tubes of said tube grouping andlaterally extending substantially to a tip diameter of said fan blades.32. A condenser according to claim 29 wherein said tubes have a widthdimension “a”, such that the ratio a/H falls within the expression0.08≦a/H≦0.25.
 33. A condenser according to claim 29 further including aprimary convoluted fin affixed to each of outermost tubes of said tubegrouping and laterally extending outwardly therefrom wherein saidprimary convoluted fin has a height dimension of “q” such that the ratioq/H falls within the expression 0.15≦q/H≦0.5.
 34. A condenser accordingto claim 33 wherein said primary convoluted fin has a fin densityfalling within the range of 8-20 fins per inch.
 35. A condenseraccording to claim 33 wherein at least a first of said tubes has aninverted U-shape having two downwardly extending legs and furtherwherein each leg has an opening for receiving into said tube a vapor ofa working fluid and for discharge of condensed working fluid.
 36. Acondenser according to claim 35 wherein said first of said tubes has awidth dimension “b”, such that the ratio b/H falls within the expression0.08≦b/H≦0.5.
 37. A condenser according to claim 35 wherein at least asecond of said tubes is positioned within said U-shape of said firsttube.
 38. A condenser according to claim 37 wherein said second tube hasan inverted U-shape having two downwardly extending legs and furtherwherein each leg has one of said openings.
 39. A condenser according toclaim 38 wherein adjacent ones of said legs of said first and secondtubes have a secondary convoluted fin extending therebetween.
 40. Acondenser according to claim 39 wherein said secondary convoluted finhas a height dimension of “p” such that the ratio p/H falls within theexpression 0.08≦p/H≦0.5.
 41. A condenser according to claim 40 whereinsaid primary convoluted fin and said secondary convoluted fin have a findensity falling within the range of 8-20 fins per inch.
 42. A condenseraccording to claim 40 wherein said primary convoluted fin has a findensity different than said secondary convoluted fin.
 43. A condenseraccording to claim 38 wherein two innermost ones of said legs of saidfirst and second tubes define an unobstructed duct therebetween.
 44. Acondenser according to claim 38 further including a third convoluted finextending between two innermost ones of said legs of said first andsecond tubes.
 45. A condenser according to claim 37 wherein said secondtube comprises a central stem tube having a single inlet opening forreceiving into said tube a vapor of a working fluid and for discharge ofcondensed working fluid.
 46. A condenser according to claim 45 whereinsaid central stem tube has a width dimension “c”, such that the ratioc/H falls within the expression 0.125≦c/H≦0.3.
 47. A condenser accordingto claim 32 further including an outer tube surrounding said tubegrouping and having an inverted U-shape with two downwardly extendingouter legs, an end of each leg having an opening for receiving into saidtube a vapor of a working fluid and for discharge of condensed workingfluid, each said outer leg affixed to an outer end of an adjacent one ofsaid primary convoluted fins.
 48. A condenser according to claim 47further including an outer convoluted fin extending outwardly from eachof said legs of said outer tube.
 49. A condenser according to claim 48wherein said outer convoluted fin has a height dimension of “r” suchthat the ratio r/H falls within the expression 0.1≦r/H≦0.2.